Living by Thoreau's Mill Brook

Purple loosestrife is back

This morning Ellen saw the first Purple loosestrife petal emerging next to the Mill Brook. Norma Farber, her mother, always enjoyed the magnificent waves of purple flowers covering the Mill Brook next to our house during her frequent visits to see her grandchildren. Back then (1970’s) purple loosestrife was not widely understood, now it is considered to be one of the seven plagues of the invasive plant family.

"Once enjoyed as a medicinal herb and as an attractive garden plant, purple loosestrife has had a destructive impact on North American wetland ecology since the early 19th century. The plants grow vigorously and spread alarmingly fast, far removed from their natural controlling agents. Infestations result in dramatic disruption in water flow in rivers and canals, and a sharp decline in biological diversity. Native food and cover plant species, notably the cattails, are completely crowded out.

Purple loosestrife may rise two metres in height and 1.5 metres in width with up to 50 erect stalks to a single woody root mass. The wide, downy and sessile leaves are 3-10 centimetres in length and opposite in two ranks. Although easily identified during its blooming season from June to September, purple loosestrife may be distinguished from similar native plants (i.e. fireweed, Epilobium angustifolium; blue vervain, Verbena hastata; blazing stars, Liatris spp. and spirea, Spiraea douglasii) by its angular stalks which are square in outline.

A single plant may produce up to three million tiny seeds annually. Easily carried by wind and water, the seeds germinate in moist soils after over wintering. Once established, infestations are extremely difficult and costly to remove by mechanical and chemical means. An alternative has been sought in biological pest control; the release of insects into North America known to feed exclusively on purple loosestrife has met with considerable success. Use of the four insect species – two beetles (Galerucella calmariensis and G. pusilla) and two weevils (Hylobius transversovittatus and Nanophyes marmoratus) provides an effective alternative to harmful herbicides in sensitive ecosystems."